netfilter: nf_nat: handle NF_DROP from nfnetlink_parse_nat_setup()

nf_nat_setup_info() returns NF_* verdicts, so convert them to error
codes that is what ctnelink expects. This has passed overlook without
having any impact since this nf_nat_setup_info() has always returned
NF_ACCEPT so far. Since 870190a9

 ("netfilter: nat: convert nat bysrc
hash to rhashtable"), this is problem.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>